240 FUNCTIONS OF THE NERVOUS SYSTEM. 



central origins of the motor, and by which they are conducted along each to 

 their opposite extremities, Physiologists have no certain knowledge. That 

 they are Electrical in their character, has been, and still continues to be, a 

 favourite theory with some; and the idea seems to derive support from the 

 marked degree in which Electricity, transmitted along the Nervous trunks, 

 can excite the changes to which those nerves are ordinarily subservient. 

 Tjjus, a feeble galvanic current, transmitted along the motor nerves of an ani- 

 mal recently killed, will call the muscles supplied by it into contraction ; whilst, 

 on the other hand, a similar current transmitted along an afferent nerve, shall 

 excite reflex movements through its ganglionic centre. Further, if the cur- 

 rent be transmitted along an afferent nerve, in a living animal, it will excite 

 sensations which are referred to the part whence the nerve arises ; and, as 

 will be shown hereafter (Chap. VI., Sect. 1), Electricity is capable of thus 

 producing sensations of a special kind, as well as those of a general nature. 

 Moreover, in the instantaneousness of the transmission of Nervous agency 

 from one part of the system to another, there is more analogy to Electricity, 

 than to any other known force. But these and similar arguments do not prove 

 the identity of Nervous agency with Electricity; since the effects of the for- 

 mer may be imitated to a certain extent, not merely by Electricity, but by 

 mechanical and chemical stimulation of various kinds. Further, there are 

 powerful arguments against such a supposition, the validity of which cannot 

 be easily set aside. All attempts to prove the existence of an Electric current, 

 in a Nervous trunk that is actively engaged in conveying motor influence, 

 have completely failed, though made with the greatest precaution. Thus, Mat- 

 teucci has lately experimented upon the very large crural nerve of a Horse, 

 which was caused, by stimulating its roots, to throw the muscles of the leg 

 into violent contraction ; nevertheless, although he used instruments of such 

 delicacy, as to be capable of detecting an infinitesimally-small disturbance of 

 the electric equilibrium, no such disturbance was apparent. Further, it is well 

 known that the conducting power of the nerves is destroyed, not merely by 

 dividing the trunk, but also by putting a ligature round it ; which last opera- 

 tion does not diminish its powers as a conductor of Electricity. Moreover, 

 the various fibrils are not as completely insulated from each other in regard to 

 Electricity, as we know them to be with respect to nervous agency ; 1'or the 

 first of these forces, when transmitted along a nervous trunk, cannot be re- 

 stricted to any fibre or fasciculus of fibres, but spreads through the entire 

 trunk, and even to the neighbouring parts in which it is imbedded; whilst the 

 latter is continually restricted to a small portion of the trunk, as is manifested 

 by its results. Again, if a small piece of nervous trunk be cut out, and he 

 replaced by an electric conductor, electricity will still pass along the nerve; 

 but no nervous force, excited by stimulus above the section, will be propa- 

 gated through the conductor to the parts below. And lastly, the conducting 

 power of Nerve for Electricity is stated by Matteucci to be not more than 

 one-fourth that of Muscle ; whilst Messrs. Toild and Bowman give it as the 

 result of their experiments, that both Nerve and Muscle are both infinitely 

 tvorse conductors than copper ; their power of conduction not ranking above 

 that of water holding in solution a small quantity of saline matter. 



a. Although, fur the sake of convenience, Electricity ami Xervous power are spoken of, 

 here and elsewhere, as actual nili/ies or agents, traveling along tlic \vhvs or cords that con- 

 duct them, it rnnst not he lorgolteii that the. present tendency <i(' M-ieiitilii: inquiry leads us 

 to abandon .such an idea, in the funner ease at li-a.-t ; what is commonly termed the trans- 

 tniision of electricity lieing the result of a molecular change, instantaneously occurring along 

 the \vhole length ol'tlie conducting body, in virtue of a diMurhaiiee, in the polar arrangement 

 of its particles, at one extremity, \vhich causes a similar disturbance to manifest itself at the 

 other. Thus if 



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